The stratigraphy and metamorphism of the Snyder Group, Labrador

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Virginia Polytechnic Institute and State University

The Snyder Group is a thin Aphebian sedimentary sequence unconformably overlying Archean rocks that has undergone deformation and a medium to high grade metamorphism as the result of the emplacement of the Elsonian Kiglapait intrusion. Five lithostratigraphic units totaling 239 m in thickness have been recognized: (1) a lower quartz arenite with local arkosic arenites, pelites, conglomerates, and a discontinuous basal conglomerate; (2) a manganiferous silicate iron formation; (3) a marble with associated calcsilicates and quartzites; (4) a sphalerite- and pyrrhotite-bearing quartz-rich, graphitic siltstone; and (5) an upper quartzite. Porphyritic andesite dikes and sills containing numerous subrounded basement xenoliths were emplaced before deformation and metamorphism. Later sills associated with the Elsonian Nain complex are also present. These intrusive rocks collectively have expanded the section to 355 m.

The lower and upper quartzites with their high maturity, festoon and planar crossbedding, ripple marks, graded bedding, and numerous conglomerates were probably deposited in a shallow marine environment under tidal influence. The fine grain size, intraformational conglomerates, small-scale crossbeds, and thin bedding of the intermediate formations suggest that they were deposited in a quiet, shallow water environment disturbed only by occasional storms. The sedimentary features and thinness but great lateral extent of the lithologies suggest that the Snyder Group is an Aphebian platform assemblage.

Three zones of metamorphic grade are defined by the aluminum silicates: I, andalusite; II, andalusite-sillimanite; III, sillimanite. Anatexis has occurred in zone III. The total width of the aureole is 2-2.5 km. Progressive metamorphic assemblages are:

metapelites: Ia - and-chl-biot-musc-qz ± Mn gar ± ksp Ib - and-cord-biot-qz ± muse ± Mn gar ± ks II - and-sill-cord-biot-qz ± muse ± Mn gar + ksp III - sill-cord-biot-qz-ksp III - alm-cord-biot-qz-ksp III - opx-alm-cord-biot-qz-ksp

metabasites: I - chl-epid-green hbl-biot-pc-qz ± cpx II - cumm-cpx-biot-pc-qz III - opx-cpx-brown hbl-pc-qz

marbles: II - chl-trem-phlog-fo-cc ± spinel II - diop-phlog-fo-cc ± spinel III - diop-fo-cc ± spinel III - periclase-spinel-fo-diop-cc

iron formation: II - Mn gar-grunerite-qz III - Mn gar-inverted Mn pigeonite-qz

Metamorphic conditions, based on aluminum silicate reactions, muscovite + quartz decomposition, experimental cordierite-garnet and pyroxene pairs, yield a temperature range in the aureole of 350° to 1000°C and a pressure of 4 ± kb. The irregular nature of dehydration and decarbonization isograds points to a large variation in fluid pressures and compositions.